Opposition activists said an air strike on rebel-held territory southwest of Damascus killed 20 people, including women and children, adding to the more than 60,000 people estimated to have been killed in the 21-month-old conflict.

Over 600,000 Syrians have fled abroad - many to neighboring Turkey, Lebanon and Jordan - as violence has spread and international efforts to find a political solution have sagged.

Refugees interviewed by the International Rescue Committee (IRC) cited sexual violence as a major reason they fled the country, the New York-based organization said in a 23-page report on the crisis published on Monday.